Electrostatic coating systems including an electrostatic atomizer, in general, are configured to electrically charge paint particles with a high voltage generated by an external or built-in high voltage generator (typically of a cascade type) such that the charged paint particles are attracted onto a work held in a ground potential. The high voltage to be applied is changed in voltage value depending upon the nature of the paint to maintain the normal voltage of the atomizer at a predetermined value (for example, −90 kV).
To keep the safety of operators, conventional electrostatic coating systems include a safety mechanism for interrupting operation of the high voltage generator and thereby stopping application of the high voltage before accidental short-circuiting occurs when the atomizer excessively approaches the work. More specifically, conventional coating systems include an overcurrent detector for detecting excessive current flowing in a high voltage cable in the atomizer. If the overcurrent detector detects a current exceeding the maximum value of the normal current (for example, 200 μA), the high voltage generator interrupts the supply of the high voltage to stop the coating operation.
However, if the interruption of the coating operation occurs during coating of a work, it will invite a great economical loss especially in case the work is an expensive product such as a vehicle body.
There are some existing electrostatic coating systems including safety mechanisms and additionally having certain means for minimizing interruption of coating operations.
One of such existing coating systems is disclosed in Japanese Laid-open Publication H9(1997)-262507. Since the leak current increases with humidity of the coating atmosphere, this prior art monitors the humidity of the coating atmosphere to lower the sensitivity of the safety mechanism. That is, when the humidity of the coating atmosphere is high, this system does not interrupt the power supply to the high voltage generator and continues the coating operation even if a current larger than the maximum normal current value flows.
Another of such existing coating systems is disclosed in Japanese Patent Laid-open Publication H2(1990)-298374. The safety mechanism for interrupting the supply of a high voltage in this coating system has an additional function of continuously monitoring the current flowing in its high voltage application path. If the safety mechanism detects a current larger than the maximum normal current value, it automatically lowers the output voltage of the high voltage generator to keep the current value within the range of the normal current.
Another of such existing coating systems is disclosed in Japanese Patent Laid-open Publication 2002-186884. This prior art remarks some problems including substantial decrease of the high voltage to be applied to the atomizer, which often occur when contamination of the atomizer by the paint or other substances increases the leak current. Thus, this prior art proposes to integrate amplitude values of the current or voltage in the high voltage application path and to generate an alarm to the operator's attention when the integrated value exceeds a preset value.
The above-introduced proposal of Japanese Patent Laid-open Publication No. H2(1990)-298374, namely, the proposal to automatically lower the output voltage of the high voltage generator upon detection of a current larger than the maximum normal current value, has the following advantage. Even when leakage of current occurs via a bridge made by a metal component contained in the paint, for example, the operator can continue the coating operation under a lower level of the high voltage applied to the atomizer and a reduced level of leak current as long as the reduced level of leak current is not likely to invite serious accidents such as fire.
Electrostatic atomizers using a rotary atomizer head typically use an air motor to drive the rotary head. Spray-type electrostatic atomizers typically use air to spray the paint. These electrostatic atomizers are subjected to leakage of electric current through dust or other contaminates in air paths. In some electrostatic atomizers having a built-in high voltage generator, the high voltage generator generates a high voltage inside the atomizer, and there is only a small distance between the high voltage generator and the rotary atomizer head (there is only a small distance of insulation). As a result, a small amount of dust or other contaminates, if any in the paint path, may become a source of leakage of electric current with a high probability. Therefore, although the coating system disclosed in Japanese Paten Laid-open Publication 2002-186884 can monitor the leak current and can generate an alarm when the leakage reaches an excessive level, it is difficult for operators to locate the very position of the leakage.